Method for increased workpiece throughput

ABSTRACT

A method is disclosed for speeding workpiece thoughput in low pressure, high temperature semiconductor processing reactor. The method includes loading a workpiece into a chamber at atmospheric pressure, bringing the chamber down to an intermediate pressure, and heating the wafer while under the intermediate pressure. The chamber is then pumped down to the operating pressure. The preferred embodiments involve single wafer plasma ashers, where a wafer is loaded onto lift pins at a position above a wafer chuck, the pressure is rapidly pumped down to about 40 Torr by rapidly opening and closing an isolation valve, and the wafer is simultaneously lowered to the heated chuck. Alternatively, the wafer can be pre-processed to remove an implanted photoresist crust at a first temperature and the chamber then backfilled to about 40 Torr for further heating to close to the chuck temperature. At 40 Torr, the heat transfer from the chuck to the wafer is relatively fast, but still slow enough to avoid thermal shock. In the interim, the pump line is further pumped down to operating pressure (about 1 Torr) behind the isolation valve. The chamber pressure is then again reduced by opening the isolation valve, and the wafer is processed.

REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. application Ser. No.09/749,648, filed Dec. 27, 2000, now U.S. Pat. No. 6,409,932 whichclaims the priority benefit under 35 U.S.C. §119(e) to provisionalapplication No. 60/194,227, filed Apr. 3, 2000.

FIELD OF THE INVENTION

The present invention relates generally to equipment and methods ofoperating equipment for semiconductor fabrication. More particularly,the invention relates to methods of improving throughput in temperatureramping.

BACKGROUND OF THE INVENTION

Photoresist removal (stripping or ashing) is one of the most frequentlyapplied processes in semiconductor industry. Due to the number ofphotolithographic mask steps in most fabrication process flows, it isimportant that ashers (strippers) attain high throughput. Removingphotoresist in a vacuum chamber using an aggressive plasma chemistry isconsidered to be the industry standard.

Two types of system architectures for plasma ashers are widely adopted.A first architecture employs a vacuum load-lock and a wafer vacuumtransfer chamber, all kept at vacuum during processing of one cassetteof wafers or other workpieces. A cassette of wafers is placed in theload-lock and the load-lock is then evacuated with a vacuum pump. Awafer transfer robot then transfers the wafers from the load-lockthrough the transfer chamber to a process chamber where plasma isgenerated to remove (strip or ash) the photoresist. In the normaloperating mode, the load-lock, the wafer transfer chamber and theprocess chamber are all constantly under vacuum. After an entirecassette of wafers is processed and transferred back to the load-lock,the cassette load-lock is then vented to atmosphere, the processedwafers are removed and a new cassette of wafers is loaded.

A second system architecture transfers wafers from the cassette in anatmosphere-to-vacuum-to-atmosphere (AVA) sequence for each individualwafer. The robot and the wafer cassette are always at atmosphericpressure. The robot transfer wafers from the wafer cassette to theprocess chamber. A vacuum pump then evacuates the process chamber to acertain vacuum level suitable for plasma formation. The plasma sourcethen generates plasma to remove photoresist. After the process iscomplete, the process chamber is vented to atmosphere and the processedwafer is transferred back into the cassette. This architecture entailspumping down the process chamber and venting back to atmosphericpressure for each individual wafer.

The first architecture employs two additional vacuum chambers (the loadlock and transfer chambers), generally requiring an additional vacuumpump to pump down two chambers, and therefore is much more expensivethan the second architecture. Machines using the first architecture areusually larger and occupy more clean room floor space, which isconsidered to be premium commodity in a semiconductor fabricationfactory. Advantageously, however, the vacuum load-lock systems of thefirst architecture have relatively low non-productive overhead,exhibiting high throughput.

On the other hand, the second technique(atmospheric-to-vacuum-to-atmospheric, or AVA) involves a lower initialcapital expenditure and occupies less space on the clean room floor. Inorder to compensate for relatively higher non-productive overhead thanthe vacuum load-lock system, a variety of improvements have been made tothe AVA system. A conventional wafer processing sequence of the AVAmachine architecture is as follows:

1. The robot transfers a new wafer or other workpiece from the cassetteto the process chamber and places the wafer on support pins.

2. The wafer is lowered onto a high temperature chuck (platen).

3. The chuck heats the wafer up to the desired process temperature (e.g.250° C.).

4. The chamber is then pumped down to a desired process or treatmentpressure (e.g. 1 Torr).

5. The process gases start to flow and plasma is ignited by a plasmasource.

6. After the photoresist has been removed, the chamber is then ventedback to atmospheric pressure (760 Torr).

7. The robot then exchanges the processed wafer for a fresh one from thecassette while transferring the processed wafer back to the cassette.

All the above steps are highly optimized to reduce the time required tocomplete each step. Heat transfer between the chuck and the wafer occursmost efficiently at atmospheric pressure; therefore, wafers are usuallyheated up before pumping down the chamber. In one commercial system, ittakes about four seconds to heat a wafer from 20° C. to 250° C., atatmospheric pressure. In contrast, at a low pressure (1 Torr), it takesabout 60 seconds.

Rapid wafer heating at atmospheric pressure sometimes causes waferwarping, however, which can have a variety of negative effects. Waferwarping may damage the circuits that are already fabricated on thewafer. If a wafer is warped on a chuck, the wafer temperature is nolonger evenly distributed. A non-uniform wafer temperature distributionresults in a highly non-uniform process, since temperature is a verysensitive process variable. The wafer is usually transferred into thechamber at room temperature (20° C.). Once inside the chamber, the waferstarts to warm up somewhat while suspended above the chuck (e.g., onlift pins). To reduce the degree of thermal shock on the wafer, andconsequently prevent wafer warping, the wafer can be left suspended overthe chuck for a few seconds to pre-heat the wafer before lowering thewafer onto the high temperature chuck. The wafer descent rate can alsobe slowed so that the wafer is warming up on its way to the chuck. Bothof these options, however, also reduce throughput.

Other efforts to improve throughput focus on minimizing the time to pumpthe chamber down to the treatment pressure (typically 1 Torr).Conventionally, a large vacuum pump is used for a high-speed pumping. Alarge diameter vacuum line is used to increase the pump lineconductance. Moreover, a dedicated roughing line, described in moredetail in the Detailed Description of the preferred embodiments, isintroduced to bypass the high resistance throttle valve and furtherincrease the overall pump speed during pump down. A dedicated roughingline is beneficial because the throttle valve can stay at its previousthrottling position, dramatically reducing the time to reach a stabletreatment pressure. Without the dedicated roughing line, the throttlevalve has to be wide open during pump-down to reduce the pump-down timeand then rotate to its throttling position when the process gases startto flow. This can take as much as an additional five seconds tostabilize the pressure after the chamber is pumped down.

Despite incremental improvements to the speed of each sequential step inthe ashing process, a need exists for further improvements to throughputfor plasma asher systems.

SUMMARY OF THE INVENTION

A method is disclosed for speeding workpiece thoughput in low pressure,high temperature semiconductor processing reactor. The method includesloading a workpiece into a chamber at atmospheric pressure, bringing thechamber down to an intermediate pressure, and heating the wafer whileunder the intermediate pressure. The chamber is then pumped down to theoperating pressure at which substrate treatment is conducted.

The preferred embodiments involve single wafer plasma ashers, where awafer is loaded onto lift pins at a position above a wafer chuck. Afterplacement and sealing the chamber the pressure is rapidly pumped downfrom a load/unload pressure (preferably atmospheric) to about 40 Torr byrapidly opening and closing an isolation valve, and the wafer issimultaneously lowered to the heated chuck. At 40 Torr, the heattransfer from the chuck to the wafer is relatively fast, but still slowenough to avoid thermal shock. In the interim, the pump line is furtherpumped down to operating pressure (about 1 Torr) behind the isolationvalve. The chamber pressure is then again reduced by opening theisolation valve, and the wafer is processed.

It will be understood, of course, that the preferred embodiments aremerely exemplary and that other intermediate pressures can be selected,in view of the disclosure herein, depending upon the system andtreatment process involved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a temperature vs. time graph, illustrating wafer temperatureramping at various pressures.

FIGS. 2A and 2B are schematic diagrams of a process chamber inaccordance with the preferred embodiments, including a wafer chuck withlift pins.

FIG. 3 is a schematic diagram of a vacuum pumping system in accordancewith one embodiment of the invention.

FIG. 4 is a schematic diagram of a vacuum pumping system in accordancewith another embodiment of the invention.

FIGS. 5A and 5B are graphs showing pressure equalization after anisolation valve is opened.

FIG. 6 is a flow chart illustrating a process sequence in accordancewith preferred embodiments of the present invention.

FIG. 7 is a flow chart illustrating a process sequence of anotherembodiment of the invention, particularly for removing implantedphotoresist.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While illustrated in the context of a single wafer, remote plasmasystem, the skilled artisan will readily find application for theprinciples disclosed herein to a variety of systems in which rapidheating of a workpiece and rapid pump-down of the a chamber are desired.The invention has particular, though not exclusive, utility for plasmaashers.

Referring initially to FIG. 1, wafer-heating characteristics werestudied at various pressure levels and it was found that the rate ofheat transfer could be controlled with pressure. FIG. 1 shows threewafer heating curves at atmospheric pressure, at an intermediatepressure and at an operating pressure. In the illustrated examples,these pressures are 760 Torr, 40 Torr and 1 Torr, respectively. Allthree curves asymptotically approach 250° C. The 1 Torr curve shows avery slow heat transfer rate. The 40 Torr curve has a slightly slowerheat transfer rate than the 760 Torr, but lags only about 2 to 3 secondsbehind the 760 Torr curve. With the chuck maintained at 250° C., thewafer temperature is approximately 245° C. at steady state when thestripping (ashing) process starts.

FIGS. 2A and 2B show a process chamber 10 in accordance with thepreferred embodiments. In the illustrated embodiments, a wafer supportstructure or chuck 12 is heated (e.g., by hot fluid circulatedtherethrough or by resistance heating) to a process temperature andmaintained at that temperature during the sequential processes describedbelow. Preferably, the temperature is greater than about 150° C., morepreferably between about 200° C. and 300° C., and most preferablybetween about 225° C. and 250° C. in the illustrated plasma ashercontext.

FIG. 2A shows a wafer 12 or other workpiece in an upper position, spacedbut proximate the heated wafer chuck 14. In the illustrated embodiment,the wafer 14 is held in the upper position by lift pins 16 that extendthrough the wafer chuck 14, although the skilled artisan will readilyappreciate that the wafer can by held by a pick-up device or othermechanism in the upper position. The wafer 14 is generally loaded intothe chamber 10 through a gate valve 18 and supported at the upperposition prior to processing, and is also unloaded from the upperposition.

The preferred system of which the chamber 10 forms a part of anatmospheric-to-vacuum-to-atmospheric (AVA) system, with attendant lowcapital costs. However, the principles and advantages disclosed hereinwill also have utility in other contexts. For example, many clustertools will have a lowered pressure for the load lock and transferchamber, and an even lower operating pressure. For such systems, theload/unload pressure may not be atmospheric, but will be higher than theoperating pressure for the treatment performed within the processchamber 10. The chamber 10 is also in fluid communication with a remoteplasma unit (not shown), though the skilled artisan will appreciate thatin situ plasma processors are also envisioned.

FIG. 2B shows the wafer 14 after being lowered to the wafer chuck 12. Inthe illustrated embodiment, the wafer 14 is lowered by withdrawing thelift pins 16, though in other arrangements a pick-up device or otherextraneous mechanism can lower the wafer to the chuck. In theillustrated embodiment, the wafer 14 is maintained on the chuck 12 bygravitation, unaided by vacuum, electrostatic or clamping forces.Accordingly, wafer curl is of greater concern.

FIG. 3 shows a high conductance vacuum system 20 in accordance with oneembodiment of the present invention. The system 20 includes a high speedvacuum pump 22. An exemplary pump is the commercially available Edwards80/1200 pump, which has a maximum pumping speed of 554 cubic feet perminute (CFM). The chamber 10 of the illustrated embodiments had a volumeof about 8 liters. Vacuum or pump lines 24 between the pump and thechamber 10 have a length of about 20 meters and a diameter of about 100mm. The volume of the pump lines 24 is significantly larger than that ofthe chamber 10 so that it draws the chamber pressure down to theintermediate pressure of the pump lines 24 quickly. Preferably, the pumplines 24 have a volume more than five times larger than the processchamber 10, more preferably more than ten times greater, and in theillustrated example the pump lines 24 are almost 20 times greater thanthe process chamber.

In accordance with the illustrated embodiment, the system 20 includes athrottle valve 26 and a throttling isolation valve 28. Additionally, aparallel roughing line 30, forming part of the pump lines 24, includes aroughing isolation valve 32. The operation of the throttle and isolationvalves 26, 28, 32 is described below.

FIG. 4 shows a vacuum system 20 in accordance with another embodiment ofthe present invention. In accordance with this embodiment, the roughingline and attendant roughing isolation valve are omitted. The remainingfeatures are similar to those of FIG. 3 and are accordingly indicated bylike reference numerals.

FIG. 5A is a high pump speed pump curve, illustrating the asymptoticnature of pressure reduction with the systems of FIGS. 3 and 4, whileFIG. 5B shows the first few seconds of the curve in greater detail. Asshown in FIG. 5A, it takes about 5 seconds to reach 1 Torr from 760Torr. The chamber pressure drops sharply once an isolation valve isopened. This is due to pressure equalization between the chamber and thepump line. In the illustrated example, the pump line is maintained atabout 40 Torr during wafer loading. When the isolation valve is opened,the chamber pressure equalizes with the larger pump line almostinstantaneously. Thereafter, in the illustrated curve, the isolationvalve is kept open and most of the pumping time is used to draw thevacuum from 40 Torr to the operating treatment pressure of 1 Torr

The invention utilizes the heat transfer characteristic at a selectedintermediate pressure (see FIG. 1) and the near instantaneous pressureequalization of the pump lines (see FIGS. 5A and 5B) to design a newprocess flow sequence that dramatically increases the machinethroughput. It also eliminates the wafer warping concern that is usuallyassociated with rapid wafer heating.

Standard Resist Stripping

FIG. 6 illustrates the process flow sequence of the preferredembodiments. The process will be first described with respect to theembodiment of FIG. 3, having the roughing line 30 and roughing isolationvalve 32 thereon. Reference is also made to the elements within theprocess chamber 10, schematically shown in FIGS. 2A and 2B. As will beappreciated by the skilled artisan, the wafer transfer robot(s), gatevalve, heating, gas flow and pressure control systems are desirablyprogrammed by software or hardwire for the process flows describedherein.

Initially, the wafer is loaded 100 into the upper load/unload positionproximate the wafer chuck (i.e., onto the extended lift pins 16 in theillustrated embodiment of FIG. 2A). During this time, the vacuum pumplines are maintained at or reduced to at an intermediate pressure,between the load pressure and the treatment pressure. The intermediatepressure is selected to balance the speed of thermal transfer betweenwafer and chuck with the speed of pump down to operating or treatmentpressure. Preferably, the intermediate pressure is between about 10 Torrand 700 Torr, more preferably between about 20 Torr and 100 Torr, and isabout 40 Torr in the illustrated embodiment.

Once the new wafer is transferred 100 into the chamber, the gate valveis closed 105, the chamber pressure is then reduced 110 approximately tothe intermediate pressure, and the wafer is lowered 115 to the heatedwafer chuck. In the illustrated embodiment, pressure reduction 110 isaccomplished by opening the roughing isolation valve 32 (FIG. 3), whilelowering 115 is simultaneously performed by withdrawing the pins 16(FIG. 2B) lower the wafer to the chuck quickly. Thus, steps 110 and 115are performed concurrently as one step 117 in the illustratedembodiment. Since the chamber pressure drops to the intermediatepressure (about 40 Torr) almost instantly, the roughing isolation valve32 is then closed again right away to maintain the chamber at a pressureof between 40 to 80 Torr for wafer heating. The time interval betweenopening and closing the isolation valve determines the chamber pressurelevel. Experiments using the preferred vacuum pumping system 20 of FIG.3 demonstrate that a 450-msec interval results in a pressure of around40 Torr. Accordingly, the roughing isolation valve 32 is preferably keptopen for less than 2.0 seconds, more preferably for about 0.5 second,and the wafer preferably lowers linearly during the same time periodfrom the upper position (see FIG. 2A) to the lower position (see FIG.2B). With the lowered pressure, thermal shock is avoided despite a rapiddescent onto the heated chuck.

In the illustrated sequence, the wafer is preferably left on the chuckand allowed to heat up during a heating period 120. The heating period120 is longer than a standard heating period at atmospheric pressure,due to the lower pressure and consequently less efficient thermalexchange, but shorter than a low pressure (1 Torr) thermal exchange.Thus, the heating period 120 is preferably greater than about 4 seconds,more preferably between about 5 seconds and 7 seconds, and is about 6seconds in the illustrated embodiment. The lengthened heating period 120desirably avoids thermal shock to the wafer and damage cause by wafercurling. The skilled artisan will readily appreciate that heating canalternatively be accomplished at the intermediate pressure by moreslowly lowering the wafer onto the chuck, such that the wafer is atprocess temperature by the time it reaches the chuck. In any case, theskilled artisan will appreciate that some amount heat is alsotransferred at other points during the process, since the chuck is heldconstant at about the process temperature. Thus, heat is transferred tothe wafer from the moment the wafer is loaded onto the lift pins in theelevated position.

During the heating period 120, the vacuum lines are evacuated by thevacuum pump and a large vacuum reservoir is again formed behind theisolation valves. Desirably, the pump lines are pumped to the operatingpressure, which is preferably less than about 10 Torr, more preferablybetween 0.6 Torr and 2 Torr, and is about 1.0 Torr in the illustratedembodiment.

After wafer heating 120, the chamber pressure is reduced 125 to theoperating process or treatment pressure. In the present embodiment,pressure reduction 125 is again accomplished by opening the roughingvalve 32 (FIG. 3) to allow the pressure equalization. Where the vacuumline 24 is at about 1 Torr and the process chamber 10 is at about 40Torr, opening the roughing valve 32 causes the chamber pressure to dropalmost immediately to about 1.7 Torr. At this time, process or treatmentgases start to flow, the throttling isolation valve 23 opens and theroughing valve 32 closes, the throttle valve 26 starts to stabilize thechamber pressure to about 1 Torr and the plasma turns on to commencewafer processing 130 (photoresist ashing or stripping, in theillustrated embodiment). All these five control actions happen almost atthe same time.

Processing or treatment 130 will be understood to refer to that portionof the sequence in which the workpiece is physically or chemicallyaltered, typically by application of process or treatment gases. In theillustrated embodiments, as noted, treatment comprises a plasma process,preferably plasma etching in the form of photoresist ashing or strippingfrom semiconductor wafer. The skilled artisan will readily appreciate,in view of the present disclosure, that the principles and advantagestaught herein will have application to a number of low pressuretreatments.

After processing or treatment 130, the chamber is vented 135 and theprocessed wafer is then removed or unloaded 140 from the chamber. A newwafer is inserted or loaded 100 into the chamber and the sequence startsagain. During the chamber venting 135 and wafer transfer 140, 100, thevacuum line is evacuated again and a vacuum reservoir is ready for thenew wafer.

Comparing the new sequence to a conventionalatmospheric-to-vacuum-to-atmospheric (AVA) sequence in the same system,the pump down time is almost eliminated. The wafer heating time isdesirably lengthened. In essence, the chamber pump-down and waferheating are accomplished concurrently. An exemplary processing sequencefor a conventional AVA process is as follows:

1. The robot takes 5 seconds to remove a processed wafer from thechamber and place a new wafer in the chamber.

2. It takes about 2 seconds to lower the wafer to the chuck to avoidwarping the wafer. It takes about 4 seconds to heat a wafer up to 245°C.

3. It takes about 5 seconds to pump the chamber down to operatingpressure and reach stabilization.

4. It takes about 15 seconds to complete the photoresist strip process,

5. It takes about 3 seconds to vent the chamber back to atmosphere.

A total of 34 seconds is required to complete a wafer. This converts toabout 106 wafers per hour throughput. With the new sequence, it takesabout 29 seconds to complete a wafer, translating into about 124 wafersper hour throughput, which is an increase of about 18 wafers per hour(17%) without any additional cost, apart from programming the system forthe specified sequence.

The process of FIG. 6, using the concept of partial pressure heating andpressure equalization, can also be applied without the use of thededicated roughing line (FIG. 4). This simplifies the vacuum systemdesign and reduces the cost of the machine.

According to this embodiment, the throttle valve 26 is initiallypositioned at the wide-open position with the isolation valve 28 closed.The wafer is loaded 100, the gate valve closed 105, and the waferlowered 115 to the chuck, as described above. The chamber pressure ispreferably simultaneously reduced 110 by opening the isolation valve 28and closing it immediately afterwards, as described with respect to theroughing valve of the previous embodiment. The desired intermediate orheating pressure (40 Torr in the illustrated embodiment) is thus quicklyattained.

During the heating period 120, the throttle valve 26 is positioned tothe preset throttling position for process condition. As in the previousembodiment, the vacuum line 24 is evacuated at the same time and avacuum reservoir is formed.

After wafer heating 120, the chamber pressure is further reduced 125 byopening the isolation valve 28 and starting pressure equalization.Process gases start to flow and the throttle valve 26 starts tostabilize the pressure. Since the pressure is equalized through thethrottle valve 26 with a preset angle, it takes about from 0.5 to 1seconds longer to reach a treatment pressure of 1 Torr depending on thepreset throttle valve angle.

After the process 130, the chamber starts to vent 135 up to atmosphericpressure and the throttle valve 26 is set to the wide-open positionagain. A typical throttle valve 26 takes about 3 to 5 seconds to go fromthe fully open position to fully closed position. By the time the newwafer is inserted into the chamber, the throttle valve 26 is fully open.

This reduced hardware setup adds about 1 second to the previous sequencein the worst case. With the same process time of 15 seconds, the machinethroughput is 120 wafers per hour.

Implanted Resist Stripping

As will be appreciated by the skilled artisan, some mask steps insemiconductor processing are utilized for implanting dopants intosemiconductor material. These implanted masks are particularly difficultto remove. In particular, as the wafer is heated, solvent in thephotoresist is volatilized but cannot escape due to the hardened crustleft on the resist surface by the implantation. The gases build up asthe wafer is heated and can explode, leaving a residue on the chamberwalls that is very difficult to remove.

Accordingly, it is advantageous to remove the crust of the resist at alower temperature (e.g., 100° C. to 120° C.), then raise the temperatureto improve throughput for the remainder of the stripping process.Unfortunately, raising and lowering the chuck temperature, whilefeasible in a radiantly heated system, reduces throughput tremendouslywith convective/conductive or resistively heated chucks.

Referring to FIG. 7, a process for removing implanted resist is shown inaccordance with a preferred embodiment of the invention. As with theprevious embodiment, the wafer is loaded/unloaded at atmosphericpressure, and processing is conducted at low pressure (preferably lessthan about 10 Torr, more preferably less than about 5 Torr; in theillustrated embodiment, preferably between about 0.6 Torr and 2 Torr,and more preferably at about 1.0 Torr). Also similar to the previousembodiment, some heating is conducted at an intermediate pressure,between about 10 Torr and 700 Torr, more preferably between about 20Torr and 100 Torr, and about 40 Torr in the illustrated embodiment.Furthermore, the chuck is maintained at about the process temperature(which is preferably greater than about 150° C., more preferably betweenabout 200° C. and 300° C., and most preferably between about 225° C. and250° C.) throughout the process described.

Unlike the previous embodiment, heating at the intermediate pressure isconducted after partial processing.

The process can be conducted with respect to either the embodiment ofFIGS. 3 or the embodiment of FIG. 4, that is, with or without theroughing line and roughing isolation valve thereon. The skilled artisanwill appreciate that the wafer transfer robot(s), gate valve, heating,gas flow and pressure control systems are desirably programmed bysoftware or hardwire for the process flows described herein.

Initially, the wafer is loaded 200 into the upper load/unload positionproximate the wafer chuck (e.g., onto the extended lift pins 16 in theillustrated embodiment of FIG. 2A). During this time, the vacuum pumplines are reduced as much as possible to an intermediate pressure,between the load pressure and the treatment pressure. Preferably, theintermediate pressure is between about 10 Torr and 700 Torr, morepreferably between about 20 Torr and 100 Torr, and is about 40 Torr inthe illustrated embodiment.

Once the new wafer is transferred 200 into the chamber, the gate valveis closed 205, the chamber pressure is then reduced 210 approximately tothe treatment pressure, and the wafer is lowered 215 to the heated waferchuck. In the illustrated embodiment, pressure reduction 210 isaccomplished by opening an isolation valve (FIG. 3 or 4), while lowering215 is simultaneously performed by withdrawing the pins lower the waferto the chuck quickly. Thus, steps 210 and 215 are performed concurrentlyas one step 217 in the illustrated embodiment. The chamber pressuredrops to the pressure of the pump lines almost instantly, but theisolation valve remains open and pumping continues until the treatmentpressure is reached (preferably less than about 10 Torr, more preferablyless than about 5 Torr; in the illustrated plasma ashing contextembodiment, preferably between about 0.6 Torr and 2 Torr, and morepreferably at about 1.0 Torr). The wafer preferably lowers linearlyduring the same time period from the upper or elevated position to thelower position (see FIGS. 2A and 2B). With the low pressure, heattransfer is slow. Accordingly, the wafer tends to reach only betweenabout 100° C. and 170° C. through the subsequent process step.

Thereafter, a stripping process 220 is conducted for removing theimplanted crust of the resist at the low temperature. In addition to aconventional plasma asher chemistry including oxidant gases (e.g., 5 slmO₂ through the remote plasma unit, carrier gas), hydrogen and/orfluorine is added. For example, 1,000 sccm of 3-15% H₂ (in N₂ or He)and/or CF₄ (1-3% of total flow) can be added to the flow through thepreferred remote plasma generator. In about 20-30 seconds, the crust isremoved and the plasma flow is stopped.

After the crust is removed, the chamber pressure is elevated 225 to theintermediate pressure between about 10 Torr and 700 Torr, morepreferably between about 20 Torr and 100 Torr, facilitating more rapidheat transfer between the heated chuck and the wafer. In the illustratedembodiment, the pressure is increased to 40 Torr, most preferably byflowing a high thermal conductivity gas and closing the isolation valve,thereby backfilling the chamber. Ideally, helium is utilized tofacilitate more effective thermal exchange. The pressure elevation canbe conducted at once or stepwise in incremental changes.

After a period sufficient to raise the wafer to the preferred processtemperature (about 245° C. in the illustrated embodiment), the chamberpressure is again reduced 235 to a treatment pressure, preferably thesame pressure as that of the implanted resist removal phase. Thestripping 240 of standard (non-implanted) resist can then proceed morerapidly at the elevated temperature. CF₄, if previously supplied, isstopped during the standard strip, although any H₂ can flow duringstandard resist stripping without any harm.

After removing 240 the remainder of the resist, the chamber is vented245 and the processed wafer is then removed or unloaded 250 from thechamber. A new wafer is inserted or loaded 200 into the chamber and thesequence starts again. During the chamber venting 245 and wafer transfer250, 200, the vacuum line is evacuated again and a vacuum reservoir isready for the new wafer.

Conclusion

We have run the new sequence and compared the process results with theconventional sequence. Both results are nearly identical. In theconventional sequence, the rate of wafer descent onto the chuck iscritical. If the rate of descent is too fast, wafers may warp. The newsequence improves confidence because wafer warping may be eliminated asa concern due to the slower rate of heating at 40 Torr. Using the vacuumreservoir and pressure equalization concept, the extensive multitaskingcontrol sequence increases machine throughput significantly for muchimproved productivity.

Although the foregoing invention has been described in terms of certainpreferred embodiments, other embodiments will become apparent to thoseof ordinary skill in the art in view of the disclosure herein.Accordingly, the present invention is not intended to be limited by therecitation of preferred embodiments, but is intended to be definedsolely by reference to the appended claims.

We claim:
 1. A method of removing implanted photoresist from aworkpiece, the method comprising, in sequence: loading the workpieceinto a process chamber at a load/unload pressure; reducing pressure inthe process chamber to a preliminary treatment pressure and removing animplanted crust from the photoresist; raising pressure in the processchamber to an intermediate pressure between the load/unload pressure anda secondary treatment pressure; reducing pressure in the process chamberto the secondary treatment pressure; and removing non-implantedphotoresist from the workpiece at the secondary treatment pressure. 2.The method of claim 1, wherein removing the implanted crust comprisesproviding an oxidant gas and a fluorine-containing gas.
 3. The method ofclaim 2, wherein removing non-implanted photoresist comprises providingan oxidant gas without a fluorine-containing gas.
 4. The method of claim3, wherein each of the oxidant gas and the fluorine-containing gas areprovided through a remote plasma generator.
 5. The method of claim 1,wherein each of the preliminary treatment pressure and the secondtreatment pressure is less than about 5 Torr and the intermediatepressure is between about 10 Torr and 100 Torr.
 6. The method of claim1, further comprising heating workpiece by lowering the workpiece to aheated support structure while reducing the pressure in the processchamber to the secondary treatment pressure.
 7. The method of claim 6,wherein raising pressure in the process chamber further heats theworkpiece until workpiece temperature stabilizes.
 8. The method of claim7, wherein the heated support structure is maintained at a temperaturebetween about 200° C. and 300° C.
 9. The method of claim 8, whereinworkpiece reaches a temperature between about 100° C. and 170° C. afterlowering the heated support structure and prior to raising pressure inthe process chamber.
 10. The method of claim 9, wherein raising pressurein the process chamber increases workpiece temperature to between about200° C. and 250° C.